Primitive Assembly is the stage in the OpenGL rendering pipeline where primitives to be rendered are sent. These primitives are broken down into individual points, lines, and triangles (eg: if a triangle strip was rendered, primitive assembly is where the strip is converted into triangles). Primitives can be discarded at this stage, either discarding all primitives or just those with a certain winding order.

Rasterizer discard

All primitives can be discarded by enabling GL_RASTERIZER_DISCARD. This prevents any primitives from being rasterized.

This is useful for testing the performance of prior rendering stages, but it is also useful to prevent rendering of primitives generated during Transform Feedback operations.